1. Field of the Invention
The present invention relates to mine roof bolts, generally to mine roof bolts constructed of multi-strand steel cable, and in particular relates to such a cable mine roof bolt that will begin to yield without breaking (i.e., extend in length under tension load) after the tension load on the bolt has reached a predetermined maximum amount.
2. Description of the Prior Art
Cable bolts are a relatively recent innovation in the art of mine roof bolts and mine tunnel roof support systems. Each of my previous two U.S. Pat. Nos. 5,230,589 issued Jul. 27, 1993, and 5,259,703, issued Nov. 9, 1993, is directed to a mine tunnel roof cable bolt comprising a length of multi-strand steel cable having a bolt head comprising a tapered compression plug that is pressed into the mating surface of a drive collar to define the bolt head.
U.S. Pat. No. 5,253,960 also discloses a cable bolt design that uses a cable bolt in a system to monitor tension load on the bolt as represented by movement of the bolt head through a mine roof plate.
PCT Published Application No. 92/00639 also discloses the use of a cable as a mine roof bolt, the bolt head being defined by a nut screwed onto threads that are cut or rolled into the outer surfaces of the cable strands.
Neither the cable bolt of my previous patents, U.S. Pat. Nos. 5,230,589 and 5,259,703, nor the cable bolt of PCT Published Application No. 92/00639 is intended to yield (extend in overall length without failure) under tension load. However, certain geological formations above mine tunnels require that the mine roof bolts in the roof support systems yield a certain amount without breaking. In this regard, the bolt of U.S. Pat. No. 5,253,960 is designed to accommodate a small amount (approximately two inches) of linear extension under tension load. This linear extension is effected by slippage of the tapered collar of the bolt head through the center hole of a mine roof plate for the length of the tapered collar, approximately two inches. After that approximate two inch movement, additional tension force will cause the bolt to fail, as in the case of non-yieldable mine roof bolts. In addition, because the bolt "extension" is provided by a tapered collar passing through and deforming the mine roof plate, the force required to effect this movement (extension) of the bolt relative to the mine tunnel roof is not constant, but rather increases at a rate which is dependent upon a number of factors - - - the taper of the bolt collar, original size of the mine roof plate center hole, thickness of the mine roof plate, material of the mine roof plate, etc. As a result, this device is practical only to measure the amount of linear displacement of the bolt head relative to the mine tunnel roof, and this linear displacement only within the length of the tapered section of the bolt head, approximately two inches. If information regarding the load force on the bolt is required, it is necessary to install a crows foot device and approximate the load using a formula that involves the linear bolt head displacement and crows foot resistance force amounts.
In some geological formations above mine tunnels, it is necessary for the roof support system bolts to yield (i.e., extend) up to four feet or more without failure. It is therefore an object of the present invention to provide a yieldable cable bolt that will yield (extend) up to four feet or more in tension without failure.
It is a further object of the present invention to provide such a yieldable cable bolt that will maintain tension up to a pre-determined amount of tension force prior to yield, thereafter maintain this yielding force (load) at a near constant value throughout the yielding process.
It is another object of the invention to provide a yieldable cable bolt which will provide a way of easily visually measuring the amount of yield (axial displacement of the bolt relative to the mine tunnel roof).